The present invention relates to an instrument for measuring the flow rate of a gaseous or liquid fluid by means of measuring the velocity of the fluid flow, more particularly, to a flow rate measuring instrument, specifically, a self-oscillation type flow rate measuring instrument, provided with a correction device capable of assuring the determination of a true flow rate from a detected flow velocity of the fluid.
Generally, in a flow rate measuring instrument based upon the measurement of flow velocity, it is essentially required that a correction be performed in order to obtain a true flow rate of the fluid from the detected flow velocity. The correction may be performed in response to the change in related parameters such as the density and surface level of a liquid to be measured.
A flow rate measuring instrument utilizing the self-oscillation was proposed and disclosed in the Japanese Patent Application No. 27927/1974 which was laid-open to the public under the Japanese Patent Publication No. 122956/1975. This self-oscillation type flow rate measuring instrument shown in FIG. 1 comprises a delta body, light in weight, made of Aluminium or other light materials and rotatably supported in a fluid flow F. The delta body 1 may be formed into any shape including an opposing face 1a facing against the flow F and a tail portion 1b extending backwardly from the face 1a along the direction of the flow F. FIGS. 1 and 1a show the delta body 1, 1' formed into a triangle and a T-shape in section, respectively. The delta body 1' comprises a plate-like opposing face 1a' and tail portion 1b'.
In FIG. 1, a reference numeral 2 designates a shaft, made of a material such as Aluminium, light in weight, and fixed to a suitable position of the delta body 1 symmetrically in such a manner that it vertically projects therefrom, a reference numeral 9 designating a U-shaped supporting frame extending in the horizontal direction and provided with bearings 3 for pivotably supporting the tip portions of the shaft 2 with reduced friction.
Furthermore, there are provided a balance weight constructed in a criss-cross fashion and fastened to the intermediate portion of one of the shafts 2, a sector 5 to be detected and secured to an appropriate portion of the shaft 2, non-contact sensor 6 for detecting the movement of the sector 5 and outputting pulse signals responsive to the movement, a relay or amplifier 7 connected to the output of the sensor 6, and a counter 8 connected to the output of the relay or amplifier 7.
The self-oscillation type flow rate measuring instrument constructed as mentioned above is arranged in the flow F so that the shaft 2 of the rotary member 1 is positioned along the direction perpendicular to the direction of the flow F. This causes the interaction between the delta body 1 and the fluid, allowing the self-oscillation of the delta body 1 around the shaft 2 to occur. The resultant oscillation of the sector 5 is detected by the sensor 6 and thus, the oscillation frequency of the delta body 1 is counted by the counter 8.
However, in case of employing such a flow rate measuring device utilizing the self-oscillation responsive to the velocity of the flow, the variations in the density and surface level of the fluid to be measured cause an error in determining the true flow rate.
Conventionally, several methods for eliminating the above-mentioned drawback have been proposed. One of the most generalized methods is based upon the utilization of a calculator which receives the flow velocity, the density and level of the fluid or the like for the calculation of the true flow rate. The method, however, can produce disadvantages of causing the necessity of employing a complicated system and being expensive in manufacturing costs.
Although methods for performing a direct correction have also been developed, these methods have seldom been utilized in practice. It can be noted that there cannot be found any simple devices capable of measuring the flow rate of a gaseous fluid in the case of large variations in pressure appearing.
Furthermore, mass flowmeters which have been proposed heretofore are also complicated in construction.
Consequently, improvement of conventional devices has been desired in the field to which the present invention pertains.